The only thought I can offer is that continuity was not as important to people then as it is now.
^ I suspect this is the truth.
What people often forget is that film was, before the advent of home video, a transient medium, one that was enjoyed for two hours and then forgotten, unless you made multiple trips to the cinema. It wasn't like television, where there were always reruns. And there was, of course, anything up to two or three years between sequels. Nobody was expected to remember the fine details for that long.
It's for the same reason that Superman II
using Kal-El's mother in the fortress of solitude scenes, repeating word-for-word dialogue said by Jor-El in the original movie, didn't actually matter at the time. Nor Carol Marcus' absence from the Genesis tape footage in ST III
for that matter, or Jennifer being recast in Back To The Future II
. Nobody was expected to remember small details like that back then. It really only stands out today because we've got used to being able to watch movies back-to-back in the comfort of our armchairs.